CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

'91 Saab 900 SPG running rich

I have a Saab 900 turbo that had been stored for seven years. It has now been awoken and has an intermittently stumbling idle with black exhaust. This only occurs when the car is warm. Otherwise it seems to run fine. The car never had this problem prior to its storage seven years ago.



The spark plugs are left with black soot which is easily brushed-away. The check engine light becomes soon after warm-up and the error codes read:



12223 lean mixture (i checked vacuum hoses and injector seals)



12213 full throttle signal at idle from throttle position switch (i checked throttle position switch function at the control unit connector)



12112 oxygen sensor self-compensating circuit problem-incorrect mixture while driving



The engine temperature sensor tests fine, as does the ELCD and AIC as per tests in Bentley. I replaced the oxygen sensor, the fuel pressure regulator, airmass meter, but the same problem remains with the same error codes.



I did notice one faulty electrical test at the control unit connector: the vehicle speed sensor gave an alternating voltage of 0.02V and 4.50V instead of 0.50V and 5.00 V as stated in Bentley. Do you think that this is significant?? There are not many parts available for Saab 900s, and the fact that this seemingly ‘random’ part is still available makes me wonder if this is prone to failure.



[…running out of cash and a bit mystified.

worse than that i am course-5 and never worked on a car newer than 1974.]

I think the circut that enriches the fuel mix for cold starts is staying active after the car has reached operating temps. Don’t know the car specifically well enough to tell you what to do about it. Last Saab owned was an '87 Turbo. Saab fuel injection systems are tricky.

Just wild guessing here a bit without car in hand, but the problem could be related to the fuel system.
Extended storage is not good on cars and this is especially true of the fuel system.

Some fuel related possibilities could be:
Low/erratic fuel pressure causing poor injector spray patterns, and in turn misfiring.
Varnished screen on the fuel pump pickups. (see above)
Plugged up fuel return line.

Given the tendency for those high powered Bosch pumps to go South after being parked for a while is my reason for suggesting a fuel system problem.

The reason why it’s worse when the engine is warm is because a cold engine will consume more fuel than a warm one and the problem may be simply covered up to some extent by the cold motor.
Based on what is apparently all of the plugs being sooted up rather than one, and considering the pump history, I’m leaning towards a pump intermittently bogging down.

Hope that guess, on or near the mark or not, helps.

I’d go back and have another look at 12213 - the throttle position switch (tps). Check the tps voltages closer to the engine computer (ecu), with the throttle being slowly moved from wot (wide open throttle) to idle.
You did check fuel pressure, at idle and with the engine under load (in drive, 2000 rpm), didn’t you?

Thanks for the helpful suggestions.

  1. I rechecked the throttle position switch signal at the control unit connector, and they are all consistent with what is expected as per Bentley manual.

  2. The fuel pressure runs a bit high at 38psi at idle (should be 35psi for a 3.0bar regulator). Manifold vacuum at idle is 18-19". Running the car under full load at 2000rpm (in order to obviate turbo boost) the fuel pressure was 48psi. [Please note that I am using a 3.0bar fuel pressure regulator whereas stock for 900turbo is 2.8bar; the car had been modified with factory APC-mod kit to allow higher boost; the car ran fine for years with this modification, right up until it was placed in storage].

–I recently checked the fuel return line and noticed that I could barely push fuel through it using exhaling pressure by mouth. I attached clear tubing to the hose, filled it with fuel, and noticed that the level dropped only ~1/4" per minute by gravity alone.

I think that this might be the problem. How do you suggest that I could unclog the return line? Is it possible that the rubber hose/tygon tubing connections could have swollen? …remember that the car was stored for seven years before this problem occured.

  1. I rechecked the throttle position switch signal at the control unit connector, and they are all consistent with what is expected as per Bentley manual.

  2. The fuel pressure runs a bit high at 38psi at idle (should be 35psi for a 3.0bar regulator). Manifold vacuum at idle is 18-19". Running the car under full load at 2000rpm (in order to obviate turbo boost) the fuel pressure was 48psi. [Please note that I am using a 3.0bar fuel pressure regulator whereas stock for 900turbo is 2.8bar; the car had been modified with factory APC-mod kit to allow higher boost; the car ran fine for years with this modification, right up until it was placed in storage].

–I recently checked the fuel return line and noticed that I could barely push fuel through it using exhaling pressure by mouth. I attached clear tubing to the hose, filled it with fuel, and noticed that the level dropped only ~1/4" per minute by gravity alone.

I think that this might be the problem. How do you suggest that I could unclog the return line? Is it possible that the rubber hose/tygon tubing connections could have swollen? …remember that the car was stored for seven years before this problem occured.

You should be able to blow somewhat easily through the return line, which should terminate in the fuel pump canister.

While working for SAAB I remember seeing a couple of return line problems but the problem on these involved the line in the tank; not the external lines from up front back to the tank. One in particular refused to clear up no matter what. No amount of banging, profanity, and compressed air would clear it out.

A restriction in the return line can definitely elevate fuel pressure and cause rich running.
Something else to consider might be a cold start valve acting up. This valve should only activate for a few seconds when the engine is cold.
A fault here could cause the symptoms the car has right now. A cold engine or one at speed may cover this up to some extent but a warm engine at idle may really act up.

You could try popping the cold start valve out after shutting the engine off and eyeballing the tip. If it appears to be wet or is dribbling gasoline out then you know there’s a problem with it.

Thanks for the suggestions…

I removed the return line from the fuel pump cannister. There is easy flow into the cannister from its inlet socket. The return line check valve seemed functional too.

The fuel pressure with pump running and engine off is 46psi. The pressure was stable and smooth. However, Bentley states it should be 43.5psi.

I checked the fuel return line function more objectively by replacing it with rubber hose from the fuel pressure regulator outlet directly to the fuel tank filler neck. The fuel pressure was still 46psi. Fuel flow was ~900cc/30secs as per Bentley. The fuel system was able to hold its pressure for hours after shut-off.

Also, running the engine at idle with the fuel return line bypassed still tripped the check engine light with eventual sooty exhaust as it warmed-up. The fuel pressure at idle is 38psi (should be 34.8psi for this regulator as per Bentley) with and without the fuel return line bypassed. So, i guess that the return line is not a culprit. And it seems that the fuel delivery checked with engine off is smooth.

This engine has an LH 2.4 fuel injection so there is no cold start injector (as the control unit is responsible for cold starts). -Bentley does not mention a cold start injector nor can I find one supplied by the fuel rail.

I can only think that maybe there is a vacuum leak causing too little vacuum to reduce the fuel pressure regulator’s pressure. However, I measure 18-19" of manifold vacuum at idle. Is this maybe not enough vacuum?

I am mystified… (BTW i am using a new Actron fuel pressure gauge …hopefully it’s reasonably accurate)

Being off a few PSI on the fuel pressure should not be the cause behind this problem and I think your Actron gauge is fine.

The manifold vacuum of 18-19" is in the normal range. The vacuum will vary a bit based on ignition timing, altitude, barometric pressure, etc. but it won’t vary that much. You should be fine as far as vacuum leaks are concerned.
The speed sensor voltage should not be a significant factor, at least in my opinion.

What about ignition timing in all of this? If the timing is retarded too much this has the effect of richening the fuel mixture to the point where the computer can’t keep up with it.
In a way, advancing the timing too much can do the same thing. Too much advance has the effect of leaning an engine out but at some point it can create enough of a misfire situation which then leads to an overly rich situation. Convoluted a bit if you think about it, but true nonetheless.

What about the possibility of a weak ignition coil? A weak spark can cause an engine to apparently run rich and soot plugs up.
The old VW air-cooled Bugs and Beetles were especially prone to this as they used resistors screwed onto the plug wires. A failed resistor, and eventually a failing coil due to the resistor, could cause them to belch black smoke, soot plugs up, and even badly dilute the engine oil with gasoline due to the misfire being so bad.

Hope some of that helps.

Thanks for the advice. This is certainly something that I had overlooked. BTW would you know how much vacuum advance '91 Saab SPG Turbos have (at idle)? I could not find this anywhere in the Bentley manual.

Thanks so much again…

The timing info should be on the underhood sticker and no matter the type of car, the hood sticker always takes precedence over anything that either myself or a manual would state.

Setting the timing on a SAAB is normally done a bit differently than other cars. On the 900 series this usually means disconnecting the advance vacuum hose and plugging it followed by revving the engine up to a steady 2000 RPMs.
The timing is then adjusted to 20 degrees BTDC. Hope that helps.

I see that the mechanical advance is 16 degrees BTDC for this car. And that is what the timing reads (with vacuum pipe plugged). I was just wondering if the vacuum advance mechanism in the distributor was sticking somehow and not allowing full vac advance. The car did sit unused for seven years. I shall certainly check the internal resistance in the coil too, however I think that the ignition output is fine since the car runs so well except only for warm idle.

The ignition coil resistances check-out fine and the ignition timing too. I just realized that the vacuum advance is ported so there is no vacuum advance at idle.

Is there anything else that you could suggest before considering to replace the LH 2.4 control unit? The only error code that disturbs me is the ‘full throttle signal at idle from throttle position switch’. However, the throttle position switch function checks-out fine at the connector at the control unit. I am wondering why the control unit decides to trigger that warning.

The only other issue is that the fuel pressure regulator was changed from the 2.8bar to a 3.0bar for an upgraded APC unit. However, the car had been running fine with this modification for years before it was commited to 7year storage.

I don’t see the 16 degrees of advance or the fuel press. reg. being the cause of this problem.
The advance vacuum is ported but the hose should always be plugged or pinched off anyway.

There is an oddball possibility behind the error code you mention. Maybe there’s a chance the wiring in the harness is degrading.
Biodegradeable wire harnesses (save the junkyard rats) being problematic is not that rare a problem on European cars of this era. Mercedes has the bulk of the complaints I think but this problem also affected other makes.

I owned an '85 900 that developed some absolutely maddening electrical quirks and it took me forever to find the cause; a degrading harness buried down low on the driver side. Testing with a VOM was pointless (no lack of continuity anywhere) and the only way I ever found it was to start splitting the harness with a razor blade. At that point I found a heck of a corroded mess.

About all I can suggest is to find a factory schematic so you will know which wires you’re working with on that circuit, disconnect them at their sources, and try to determine if they’re shorted to each other or bleeding over into another circuit.
I don’t know if this is the cause of your car’s problem or not; just pointing out a possibility. Hope this helps.